1. Field of the Invention
The present invention relates to a hot air generator/burner apparatus with an extender which may notably but not exclusively be used for low temperature heating of a plastic film, for example a polyethylene film, with a view to their retraction by means of a gas flow stemming from the combustion of a combustible gas such as propane and air or for laying a thermoplastic material on the ground.
2. Description of the Prior Art
In order to satisfy this type of application, the burner should therefore be designed in order to produce a gas flow having a temperature of 120°-540° C. at a predetermined distance from the burner (a distance at which the material to be treated should be positioned).
At this distance, the temperature must be relatively homogeneous and the gas flow free of combustion material, if the intention is to avoid any possibility of burning, scorching and blistering of the material.
In order to achieve this result, a burner has already been proposed, particularly in the U.S. Pat. No. 6,010,329, comprising an injection device capable of producing a high velocity flow of combustible gas mixture and of injecting this flow into a burner head with a tubular shape successively including:                a pressure recovery chamber having, in the plane of symmetry of the head, a divergent shape and inside which the gas mixture stemming from the injection device develops according to a fan configuration,        an ignition chamber with a substantially constant and rectangular section,        two deflectors which respectively extend the two major sides of the ignition chamber, and which converge towards each other, both of these deflectors having two rectilinear front borders forming a passage with reduced width between them, and        diffusion means which may comprise a grating or even a set of two gratings, with a substantially hemi-cylindrical shape, attached to the inside of the head at the junction of both chambers, these diffusion means forming a bulging partition, with an axis parallel to said borders, in said plane of symmetry, and the concavity of which is directed towards the inside of the pressure recovery chamber.        
In this burner, the diffusion means may be produced from a grating or a perforated metal sheet.
It is found that by means of the structure described earlier, the burner head is not licked by the flame and therefore does not undergo significant heating.
In order to homogenize the anterior front of the flame and avoid that untimely orientations of the burner induce heterogeneities of the flame, the use of circular shapes for the front borders of the ignition chamber and deflectors as well for the diffusion grating was proposed (Patent Application FR 87 06930).
The invention more particularly relates to a burner of the type of the one described earlier equipped with an extender consisting in a tubular, possibly telescopic, component, which may be inserted between the tubing of the injection device and the head of the burner.
With Patent Application PCT/FR05/02720, the Applicant has already proposed a solution for solving the following problems:                A first problem resulting from the fact that upon extinguishing the burner, a relatively large volume of gas mixture remains inside the assembly formed by the head of the burner, the extender and the injection tubing. Now, upon stopping the burner, the flow rate of the gas mixture, notably through the diffusion grating, is lowered before becoming zero. Therefore, below a certain flow rate, diffusion means, the perforations of which have been determined so as to obtain in the ignition chamber a large gas flow at a relatively high velocity, no longer retain the flame. This is the reason why the combustion propagates inside the aforesaid assembly by generating a slight explosion. This explosion which is not devoid of any risk has especially the drawback of being noisy, and consequently difficult to accept in a factory or on a building site.        A second problem resulting from the fact that the extender is made in an electrically conducting material and is connected to the ground of the electric (piezoelectric) generator. Therefore, the conductor which passes in the extender in order to connect the output of the electric generator to the ignition electrode positioned in the ignition chamber, forms with said extender a capacitor, the capacitance of which depends on the length of the extender and on the positioning of the conductor inside said extender. This capacitor has the drawback of absorbing a significant fraction of the electric charge delivered by the electric generator upon ignition. The charge applied to the ignition electrode is therefore lowered.        A third problem results from the fact that the gases propelled to a high velocity by the injection tubing do not mix homogeneously inside the extender. This heterogeneity is itself dependent on the length of the extender. The use of deflectors intended to generate perturbations in the gas flow in order to improve its homogeneity however has the drawback of slowing down the flow which is contrary to the sought effect in a high velocity burner.        
Nevertheless, the conducted tests have shown problems concerning how these burners are used, taking into account their great length and the distribution of their masses.
Thus, notably for carrying out applications of hot-melt materials at ground level, first of all pistols equipped with a straight extender not exceeding 600 mm were suggested, vertically held at arm's length, the burner being held at a determined distance from the ground, for example at 5 cm from the ground.
It is found that this solution has the following drawbacks:                The operator is forced to hold the burner at arm's length in order to move the heat source away from his/her feet, whence tiredness (the duration of use may amount to 50% of the work duration).        It is impossible to work in an area away from the operator.        When the device is stopped, the operator has to hold the handle very tightly in order to maintain it, otherwise, he/she risks performing an action on the trigger for opening the gas flow.        The length of the extender does not suit all the heights of users and makes the use of the burner even more tiresome.        The operator is forced to hold the burner with one hand and the gas supply flexible hose with the other hand so that he/she is not in the working area.        It is very difficult to maintain the head of the burner with its axis perpendicular to the plane of the surface of the object to be treated so that the action of the burner is not homogenous and a risk of burning oneself is incurred.        